1. Field of the Invention
This invention relates to an automatic transmission for use in a vehicle.
2. Description of the Prior Art
As is well known, a gear train in an automatic transmission for use in a vehicle is constituted by combining a plurality of planetary gear sets, each of which is mainly provided with three components consisting of a sun gear, a ring gear and a carrier for holding a pinion gear meshing with the sun gear and the ring gear. Namely, the planetary gear set holds either one of the three components stationary, and serves the remaining two components as an input component and an output component to perform the overdriving, retarding and reversing operations. Also, either two of the three components are interconnected to rotate the whole planetary gear set as a single unit. Therefore, the predetermined components themselves of a plurality of planetary gear sets are constantly interconnected or selectively interconnected through clutch means, and the remaining predetermined components thereof are held stationary, so that the rotational speed of an input shaft is changed on multiple stages to be transmitted to an output shaft. Thus, the more the number of the planetary gear sets used in the automatic transmission is, the more the number of gears capable of being set is, so that the automatic transmission improves in power performance and start acceleration property of the vehicle as a whole. However, with the increase of the number of planetary gear sets, the number of necessary engaging means such as a clutch and a brake is increased, whereby the automatic transmission becomes large-sized and increases in weight. Since the automatic transmission is generally accommodated in an engine compartment together with an engine, the automatic transmission requires that the size and weight are respectively as small and light as possible to such an extent that the resultant automatic transmission fulfills the properties necessary for the vehicle, so that the automatic transmission for use in a passenger car for example is constituted so as to set the gears with three forward speeds or four forward speeds including over-drive and one reverse by the use of two or three planetary gear sets.
Among the conventional automatic transmissions, the automatic transmission, which sets four forward speeds including the over-drive by using three planetary gear sets, is generally constituted such that one planetary gear set for so-called over-drive is added to a gear train consisting of two planetary gear sets constituted to set three forward speeds and one reverse, whereby such automatic transmission outputs the rotation transmitted by the two planetary gear sets and overdriven by the planetary gear set for the over-drive. Thus, this automatic transmission reduces in rotational frequency of an engine in travelling at high speed to improve in the silencing property, whereas it does not particularly contribute to the improvement on the power performance of a vehicle, since the width of change gear ratio (ratio of change gear ratio of first gear to that of the highest gear) is not particularly expanded.
Recently, the automatic transmission using three planetary gear sets is constituted such that all three planetary gear sets are constituted so as to participate in the setting of the gear even in the gears of low and medium speed regions. Hence, it is attempted that more gears (for example, five forward speeds and one reverse) are set to expand the width of the change gear ratio.
Since the automatic transmission, in which the first gear is set by compositely acting the all three planetary gear sets, provides a larger value of the change gear ratio, the width of the change gear ratio is expanded to improve in the power performance of the vehicle, and besides, the start acceleration property. However, in the second, third, fifth or reverse gear, there is the case where any of the planetary gear sets does not participate in the transmission of torque for setting the corresponding gear on which the planetary gear set will be subjected only to idle running. Then, since the relative or absolute rotational frequency of either component in the planetary gear set may be increased, and the relative rotational frequency of the pinion gear held by the carrier to the carrier may be increased. This is derived from the contradictory requirements for increasing the number of gears to be permitted to be set and reducing the weight and size of the automatic transmission, so that the engaging means such as a clutch and a brake is not generally provided between the components themselves, which do not need to be disconnected from each other in setting the gear, and a fixed portion such as a casing. As a result, since the conventional automatic transmission causes the components to be subjected to idle running with high speed on a predetermined gear, the conventional automatic transmission degrades the durability of bearings, and brings about the loss of power and the rise of oil temperature caused by the unnecessary agitation of lubricant oil.